Coaxial cable whose dielectric partly consists of air

ABSTRACT

A longitudinally watertight coaxial cable whose dielectric consists partly of air, including spacer filaments of synthetic resin which are located about the central conductor in accordance with spatial wavelines and which are interconnected at regular spaces over a short distance, and thus jointly surround the entire circumference of the conductor.

0 United States Patent [191 [111 3,864,509 Gommans 14 1 Feb. 4, 1975 1 1 COAXIAL CABLE WHOSE DIELECTRIC {56] References Cited PART" CQNSISTS OF AIR FOREIGN PATENTS OR APPLICATIONS [75] Inventor: Hubertus Johannes Josephus 907,582 3/1946 France 174/28 Gommans, Venlo, Netherlands 460,211 1/1937 Great Britain 174/28 [73] Assignee: Us Philips Corporation New 653,669 11/1937 Germany 174/24 York Primary Examiner-Arthur T. Grimley [22] Filed: Jan. 4, 1974 Attorney, Agent, or Firm-Frank R. Trifari [21] Appl. No.: 430,789

[57] ABSTRACT A longitudinally watertight coaxial cable whose dielec- [30] Foreign Apphcatwn Pnomy Dam tric consists partly of air, including spacer filaments of Feb. 2,1973 Netherlands 7301495 Synthetic resin which are located about the central conductor in accordance with spatial wavelines and [g2] :J.S.CCll 174/28, 174/36, 174/107 which are interconnected at regular spaces Over a lat. Short distance, and thus j y Surround the entire [58] Field of Search 174/28, 29, 16 B, 99 R, cumference of the conduct 4 Claims, 6 Drawing Figures P/JENTED 41975 3.864.509

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COAXIAL CABLE WHOSE DIELECTRIC PARTLY CONSISTS OF AIR The invention relates to a coaxial cable having a dielectric which consists partly of air, in which filaments of electrically insulating material serving as spacers surround the central conductor. in coaxial cables of this type a tube of electrically insulating material may be present to support the outer conductor. The cable may have one or more concentric outer conductors and furthermore an outer jacket of electrically insulating material and optionally an armor.

In a known embodiment of a coaxial cable of this type one or more filaments of synthetic resin spirally surrounding the central conductor serve as spacers. in this case the central conductor with the spacer is located within a tube of synthetic resin which tube is surrounded by the outer conductor.

Coaxial cables having filaments of synthetic resin serving as spacers spirally surrounding the central conductor can be manufactured in a continuous manner, that is to say, while moving the core at a constant rate the spacers and the further cable sections can successively be provided. A drawback of this construction is, however, that the cable is not longitudinally watertight. Consequently, when leakage occurs, for example, due to a crack or break or at a connection, water may move unhampered over large distances through buried cables and cause discontinuity in the impedance and increase the damping of the cable. The properties of the cable thereby change over large distances in such a manner that the cable becomes unfit for use. if parts of a cable section have different temperatures, water vapor may condense in the part having the lowest temperature so that the electrical properties of the cable also are detrimentally influenced. This risk occurs, for example, in cable sections which are located partly within and partly outside a building.

An object of the invention is to provide a construction for a coaxial cable which is longitudinally watertight and can be manufactured in a continuous manner. According to the invention a coaxial cable satisfying this object has spacers consisting of at least two filaments of electrically insulating material which surround the central conductor in accordance with spatial wave lines and are interconnected at regular spaces over a short distance. In this manner the filaments jointly surround the entire circumference of the central conductor.

in this connection a spatial wave line is understood to mean a wave line which varies along and around a cylindrical surface preferably between two generatrices of a cylinder. Such a wave line is three-dimensional. Each of the filaments thus surrounds only part of the conductor circumference as seen in a projection on a face at right angles to the central conductor. In the cable according to the invention the filaments jointly surround the entire circumference of the central conductor.

According to a further preferred embodiment of the invention the spacers consist of two filaments which are located along the central conductor in accordance with spatial wavelines which are symmetrical with respect to a plane passing through the axis of the central conductor. The filaments each surround half the circumference of the cylinder constituted by the central conductor, the filaments being connected together at the areas where they touch each other.

In the construction of a coaxial cable according to the invention the space between the central conductor and the outer conductor or the tube of electrically insulating material which is surrounded by the outer con ductor is divided into a plurality of compartments bounded by filament pieces. Consequently water cannot move over large distances when leakages occur along the central conductor. The formation of disturbing quantities of water due to condensation of water vapor at areas of low temperature is prevented because the flow of water vapor to these areas is prevented.

The filaments serving as spacers in the cable according to the invention preferably consist of a thermoplastic synthetic resin having a low dielectric constant (E) and a loss angle (1,, 5). Suitable synthetic resins are. for example, polypropylene. polyethylene. polystyrene. and polytetrafluoroethylene. The filaments may be solid or hollow. lt is alternatively possible to use filaments of synthetic resin foam or of a composite structure (for example, with a core of synthetic resin foam surrounded by a solid jacket of the same or of a differ ent synthetic resin). The filaments may have round, elliptical, rectangular or other cross-sections favourable for the envisaged object. It may be advantageous or desirable to deform the filaments when they are connected together in such a manner that the space between the central conductor and the outer conductor or tube of electrically insulating material carrying the outer conductor is filled up as completely as possible at the area of connection. Particularly a moisture-tight engagement of the filaments to metal parts such as the central conductor is desirable.

The spacers according to the invention may be provided in different manners, for example, with the aid of two molding devices which carry out semicircular movements around the continuously moving central conductor while at the points of return of the movement the filaments which are still hot are melted together, if desired. under deformation by means of a device which moves along with the central conductor over a suitable distance.

The central conductor may be manufactured, for example, of copper, aluminum or copper-clad aluminum. The outer conductor may likewise consist of these materials in the form of a braid of metal wires or a foil or a combination of both. The outer conductor may be supported by a tube of synthetic resin for which the above-mentioned synthetic resins are likewise suitable. it is alternatively possible to give the outer conductor the form of a self-supporting tube.

Some embodiments of a cable according to the invention will further be described with reference to the accompanying drawing.

FIG. I is a side elevational view of a central conductor provided with two filaments of synthetic resin serving as spacers.

FIG. 2 is a top view of the same part of the central conductor and filaments as in FIG. I after it has been rotated about the central axis.

FIG. 3 is a cross-sectional view of the device of HO. 2 at the area of a connection of the two filaments of synthetic resin.

FIG. 4 is a developed view of the spacers after a cut has been made parallel to a generatrix of the cylinder constituted by the central conductor.

FIG. 5 is a developed view of a spacer composed of 4 filaments.

FIG. 6 is a developed view of a spacer composed of 3 wire filaments.

In FIG. I there is shown a central conductor 1, for example, of solid copper having two spacers, of which only the spacer 2 on the front side is visible. The spacers in the embodiment shown consist of solid filaments of polyethylene.

FlG. 2 is a top view of the central conductor 1 and the spacers 2, 3, showing that the filaments 2 and 3 are connected together while being melted and deformed at 4, 5, 6 and 7. in this FIGURE the spatial character of the wavelines constituted by the filaments 2 and 3 can be seen.

FIG. 3 shows a cross-section of a complete cable taken on the line lll-lll of FIG. 2. Corresponding reference numerals have the same significance as in the previous FIGURES. A tube 8 of a synthetic resin, for example, consisting of the same material used for the wires filaments 2 and 3, surrounds the filaments. An outer conductor 9 surrounds the tube 8. This conductor may consist of a braid of metal wire such as copper wire, a metal tube, a metal foil folded about the tube 8 or a combination of a metal foil folded about the tube 8 and a braid of metal wire provided thereon. The filaments 2 and 3 are jointly deformed at the areas 4 and 6 where they are melted together such that at that area the space between the central conductor 1 and the tube 8 is completely filled up. The entire cable is provided with the outer jacket 10.

For the purpose of clarification FIG. 4 shows the spacer after the connections between the filaments 2 and 3 are interrupted on one side of the central conductor (such as between 4' and 4" and between 5' and 5"). The references 6 and 7 denote the connections located behind the central conductor 1 in FIG. 2.

FIGS. 5 and 6 show spacers in a corresponding manner which are built up of 4 and 3 filaments, respectively, of synthetic resin.

Preferably, however, the minimum number of filaments sufficient to permanently center the central conductor 1 within the tube 8 will be used. In practice this can generally be achieved with two filaments. The region of air in the space between the central conductor 1 and the tube 8 is then as large as possible for a given filaments diameter and wavelength of the wavelines constituted by the filaments resulting in a minimum damping in the cable.

In a given embodiment the central conductor 1 consisted of copper and had a diameter of 3.6 mm. Two filaments 2 and 3 of polyethylene were provided on the conductor 1 each with a diameter of 3mm and the tube 8 of polyethylene had a wall thickness likewise of 3 mm. The outer conductor consisted of a copper foil folded about the tube 8 on which foil a copper wire braid was applied. The outer jacket l consisted of polyethylene mixed with carbon black, and having a wall thickness of 2 mm.

The distance between two connection points 4 and 5, and 6 and 7, was approximately 2.5 cm. This means that when a free space bounded by the filaments 2 and 3 and two connections between these wires located on the same side of the central conductor is completely filled up with water there is no noticeable increase in damping in the frequency range between l0 and L000 MHz.

A coaxial cable according to the invention has the advantage that the construction permits continuous manufacture while the cable is sufficiently longitudinally watertight to be used outside buildings and in a humid environment.

What is claimed is:

l. A coaxial cable having a dielectric which consists partly of air in which filaments of electrically insulating material serving as spacers around a central conductor, wherein the spacers consist of at least two filaments of electrically insulating material which surround the central conductor in accordance with spatial wavelines and which are interconnected at regular spaces over a short distance and thusjointly surround the entire circumference of the central conductor.

2. A coaxial cable as claimed in claim I, wherein the spatial wavelines are symmetrical with respect to a plane passing through the axis of the central conductor, and each filament surrounds the cylinder constituted by the central conductor over half its circumference, the filaments-being connected together at the areas where they touch each other.

3. A coaxial cable as claimed in claim I, wherein the filaments, consisting of thermoplastic synthetic resin, are deformed at the area of the connection so as to fill up the space between the central conductor and an outer conductor or tube of electrically insulating material which is surrounded by an outer conductor at the area of connection.

4. A coaxial cable as claimed in claim 1, wherein said filaments are formed of a water impermeable material, said cable comprises in addition a tube of water impermeable material surrounding said spacer filaments and in water-tight contact therewith, said filaments at the areas of interconnection being shaped so as to form a water-tight seal between said tube and said central conductor, whereby said cable is made longitudinally water-tight. 

1. A coaxial cable having a dielectric which consists partly of air in which filaments of electrically insulating material serving as spacers around a central conductor, wherein the spacers consist of at least two filaments of electrically insulating material which surround the central conductor in accordance with spatial wavelines and which are interconnected at regular spaces over a short distance and thus jointly surround the entire circumference of the central conductor.
 2. A coaxial cable as claimed in claim 1, wherein the spatial wavelines are symmetrical with respect to a plane passing through the axis of the central conductor, and each filament surrounds the cylinder constituted by the central conductor over half its circumference, the filaments being connected together at the areas where they touch each other.
 3. A coaxial caBle as claimed in claim 1, wherein the filaments, consisting of thermoplastic synthetic resin, are deformed at the area of the connection so as to fill up the space between the central conductor and an outer conductor or tube of electrically insulating material which is surrounded by an outer conductor at the area of connection.
 4. A coaxial cable as claimed in claim 1, wherein said filaments are formed of a water impermeable material, said cable comprises in addition a tube of water impermeable material surrounding said spacer filaments and in water-tight contact therewith, said filaments at the areas of interconnection being shaped so as to form a water-tight seal between said tube and said central conductor, whereby said cable is made longitudinally water-tight. 